Welcome to the site, invaderjb!
It sounds like from what you describe, 7mph that is, would be around the top speed of the machine. Unfotunately riding mowers are governed to slower speeds for safety reasons.
I looked up the model number and it displays the machine as a hydrostat. There could always be the possibility that a belt is slipping or something inside the transmission is not working correctly, although you would've probably noticed the latter of the two. If it were me and I felt like something just wasn't right, I would securly lift the tractor up, check belt tension and maybe replace/flush the transmission fluid as well.